冻融低剪跨比RC梁抗震性能试验研究

通过人工气候模拟实验室对6榀剪跨比为2.6的钢筋混凝土(Reinforced concrete, RC)梁试件进行加速冻融循环试验,继而对其进行拟静力加载试验,根据试验结果分析了冻融循环作用和混凝土强度变化对RC梁试件破坏形态、滞回曲线、承载能力、变形能力和耗能能力等抗震性能指标的影响。结果表明:冻融后混凝土抗压强度降低,内部孔隙率变大,微裂缝增多,梁试件表面出现裂缝。各梁试件在拟静力加载试验后均发生了弯剪破坏。随着冻融循环次数的增加,试件的承载能力与耗能能力逐渐退化,延性先略微增长后显著下降;随着混凝土强度等级的提高,试件受冻融循环作用造成的损伤程度有所减轻,各试件的屈服、峰值、极限承载力均有所增大,耗能能力增强,延性无明显变化。     

变电站绝缘子抗震性能试验及数值模拟研究

作为变电站重要基础设施之一,变电站绝缘子的抗震性能直接影响了电力系统的可靠性。采用低周往复加载试验,从高度、轴压比及绝缘子拼装数量三个方面探讨了变电站绝缘子的抗震性能,对比分析了各绝缘子试件基于上述参数下破坏特征、滞回曲线、变形能力、耗能能力和承载能力等抗震性能指标。试验结果表明,绝缘子试件在低周循环加载试验后均发生了弯曲破坏。随着绝缘子试件高度和绝缘子拼装数量的增加,其耗能能力与承载能力逐渐减弱。随着轴压比的增加,滞回曲线的饱满程度和累积耗能能力显著降低,承载能力逐渐增强。基于实验建立的精细化有限元模型与试验结果进行了对比从而验证了模型的可靠性,为本课题后续电力系统抗震性能分析提供有效依据。     

预应力自复位混凝土框架节点抗震性能数值模拟

预应力自复位混凝土框架结构(简称PTED结构)具有独特的受力性能。为了研究其抗震性能,基于弹塑性有限元分析程序OpenSees,提出PTED节点的数值模拟方法,并利用编制的角钢拉压恢复力模型,对5个PTED节点的低周往复荷载试验进行数值模拟并与试验结果对比。研究结果表明:通过合理的建模和参数选择,采用OpenSees有限元分析程序可以较准确地模拟出试件试验过程的滞回曲线,数值模拟所得的各极限状态点对应的荷载及位移与试验结果较吻合,同时可准确地模拟出角钢、预应力筋及梁端截面的力学行为。采用该文方法进行的数值模拟与试验结果吻合较好,精度可满足工程需求。     

基于深度学习的RC梁集中塑性铰模型参数研究

集中塑性铰模型常被用于基于构件的结构弹塑性分析,计算精度依赖于模型参数的选取,但经验公式难以表征构件受力特性与模型参数的复杂非线性关系。该研究收集低周往复加载RC梁试验数据。建立构件试验数据库,采用捏拢型(pinch Ibarra-Medina-Krawinkler,Pinch-IMK)本构建立基于深度学习的RC梁构件集中塑性铰模型参数预测模型。基于试验数据对三折线骨架特征点参数、本构滞回参数进行参数辨识,得到182组骨架特征点参数数据和91组滞回参数数据;以构件特征参数为输入,以骨架特征点参数、滞回参数为输出,建立Pinch-IMK集中塑性铰RC梁构件参数深度学习预测模型HDLM。将HDLM预测骨架特征点参数与截面分析及现有经验公式等方法的计算结果作对比,可见HDLM预测结果有更高的精度;将基于HDLM预测参数计算的滞回曲线与基于经验公式的IMK模型计算结果进行对比,可见HDLM预测滞回曲线更为准确,能够较好地表现RC梁的强度退化、刚度退化和捏拢效应。     

震损RC柱FRP加固后抗震性能数值模拟分析

使用有限元软件ABAQUS,基于箍筋约束混凝土、钢筋以及FRP材料的材料本构模型,完成了未震损未加固、未震损FRP加固和震损后FRP加固的RC柱抗震数值模拟分析。对震损后FRP加固的RC柱,完成不同程度震损模拟;使用生死单元法实现严重震损混凝土的剔除与替换以及FRP的加固;再模拟修复加固后RC柱的加载,模拟结果与试验结果基本吻合。对比了3种RC柱抗震性能的数值模拟结果,发现严重震损后用FRP加固的RC柱其抗侧刚度与承载力不能完全恢复,需要考虑不同损伤程度与不同FRP加固措施等对加固效果的影响。研究了不同损伤程度和加固方法下的RC柱,结果表明：对于中等震损的试件,纵横向FRP组合加固可以完全恢复并超越原有抗震性能;对严重损伤RC柱,外围损伤严重混凝土的修复与替换是影响震损RC柱加固后性能的关键;相较于仅有横向GFRP环向加固,纵横向FRP组合加固的效果更好。     

RC梁构件基于能量的抗震设计方法研究

为研究RC梁构件基于能量的抗震设计方法,需建立一个合理的损伤指数来量化损伤。课题组前期建立了RC梁构件耗能能力与位移幅值、累积耗能和设计参数的量值关系,并提出了RC梁构件基于耗能能力的损伤指数和性能指标限值。该文在既有研究基础上提出RC梁构件基于耗能能力损伤指数的抗震设计方法。研究表明：该抗震设计方法与结构设计参数和地震参数建立了量值联系,从而便于指导结构设计;配箍率的增加可以降低RC梁构件的损伤,减损效果先急后缓;持时的增加可加剧RC梁构件损伤的发展,增加效果先快后慢;配筋率的增加可以从整体上降低RC梁构件的损伤;该抗震设计方法可以弥补现行建筑抗震设计规范中未能考虑持时效应的不足。     

配有RPC预制管混凝土组合柱的RC框架抗震性能分析

在活性粉末混凝土(Reactive powder concrete,RPC)预制管内浇注混凝土,并在RPC预制管和内部混凝土中均配置纵向钢筋,形成一种新型组合柱——RPC预制管混凝土组合柱。采用平截面假定,并结合已有的RPC、混凝土和钢筋材料本构模型,推导了该新型柱大偏压受力状态的承载力计算式。分别采用该承载力计算式和Opensees纤维模型对两种截面的新型柱承载力进行了对比分析,两种方法的计算结果较为接近。基于柱梁承载力比、RPC预制管中钢筋等级等参数,采用Opensees纤维模型分析了配有该新型柱的RC框架的抗震性能。结果表明:当RPC预制管中配置的钢筋等级提高至HRB400及以上时,结构耗能能力和延性系数均有较大程度的提高;当柱梁承载力比不超过1.2时,随柱梁承载力比的增加,结构抗震性能改善效果不明显,而当柱梁承载力比为1.5及以上时,结构抗震性能随柱梁承载力比的增加会有明显的提升。     

带施工缝RC框架结构抗震性能的数值研究

对7度、8度、9度区的规则RC框架结构进行非线性动力分析,数值模型分为在各层柱底加入施工缝模型的“带缝框架”和不考虑施工缝影响的“整浇框架”两种。通过对比结构的顶点最大位移、层间位移角、塑性铰分布规律和关键构件的反应等研究施工缝对框架结构抗震性能的影响程度。结果表明,在多遇地震下,结构仍处于弹性状态,施工缝对结构的顶点位移及层间位移角等影响不大。在罕遇地震下,随着地震动增大,结构进入非线性,施工缝的影响逐渐凸显,会使顶点位移及层间位移角明显增大。施工缝使柱端更容易出现塑性铰,更易形成“强梁弱柱”的柱铰破坏模式。在罕遇地震下对结构进行非线性数值分析时应重视施工缝的影响作用。     

钢管混凝土柱-钢筋混凝土梁连接节点抗震性能的机理分析

钢管混凝土结构因其具有良好的延性等抗震性能,在地震荷载作用下,具有较强的抗倒塌能力,使其在工程实践中得到广泛的应用。该文在已有的有限元数值分析模型基础上,通过将不同轴压比下方形和圆形钢管混凝土柱-钢筋环绕式钢筋混凝土梁连接节点的计算滞回曲线、骨架线与试验滞回曲线相比较,验证有限元模型并揭示节点的抗震特性。通过对典型轴压比下钢管混凝土结构节点的工作机理分析,研究受力全过程中节点裂缝和变形发展过程,明确节点极限状态和破坏模态;揭示节点核心区混凝土约束力、钢筋应变、核心区剪力的变化规律。基于比较不同轴压比下节点极限状态的核心混凝土应力和核心区剪力状态,确定轴压比对节点破坏模态的影响。     

结构抗震试验方法综述

反力梁是土木工程试验中的主要反力装置。当反力梁与试件的刚度比较小时,反力梁产生的变形与试件相比不可忽略。这种情况下,会对试验结果产生较大影响,特别是对拟动力和混合试验需要位移在线更新的试验影响更大。然而,关于反力梁变形的影响和补偿方法鲜有详细报道。为此,该文通过理论分析讨论了反力梁变形对抗震试验各参数的影响规律,建立了加载过程中对反力梁变形的在线补偿方法,并通过试验进行了验证。     

Failure analysis of RC beams subjected to shear through different numerical approaches

This work discusses and compares different numerical approaches that can be adopted for the analysis up to failure of reinforced concrete beams, also when they experience a brittle shear collapse. Since the development of inclined shear cracks causes a variation of the strain field normal to the element axis, as well as of the shear strains in the beam depth, this type of problem is often dealt with refined bi-dimensional nonlinear finite element analyses. The effectiveness of this type of simulations is in turn mainly related to the adoption of a sound constitutive law for the material. This work highlights that, given the same material model, also more “traditional” approaches, based on sectional analysis or on 1D finite element simulations, can be satisfactorily applied to study the problem, if their kinematic assumptions are improved and extended. In these cases, a subdivision of beam depth into several layers is also recommended. In fact, this allows to both simulate the actual position of steel reinforcement, and to widen the applicability of the method to elements characterized by a generic cross-section shape.     

A numerical approach to modelling size effects on the flexural ductility of RC beams

The problem of evaluating the rotation capacity of reinforced concrete (RC) beams in bending has been largely investigated from both the experimental and the analytical point of view during the last decades. Since the development of ductility is influenced by several design parameters, it is difficult to develop a predictive model that can fully describe the mechanical behaviour of RC beams. In particular, the role of the size-scale effect, which has been evidenced by some experimental tests, is not yet completely understood. One of the main reasons is the inadequacy of the traditional models based on ad hoc stress–strain constitutive laws. In the present paper, a new model based on the concept of strain localization is proposed, which is able to describe both cracking and crushing growths in RC beams during the loading process. In particular, the nonlinear behaviour of concrete in compression is modelled by the Overlapping Crack Model, which describes the strain localization due to crushing by means of a material interpenetration. With this algorithm in hand, it is possible to effectively capture the flexural behaviour of RC beams by varying the reinforcement percentage and/or the beam depth. An extensive comparison with experimental results demonstrates the effectiveness of the proposed approach.     

Performance of CFRP-strengthened RC beams subjected to repeated loads

In this paper, the effects of fatigue leading to crack formation and potential durability-bonding problems in reinforced concrete (RC) beams strengthened by carbon fiber reinforced polymer (CFRP) are studied. These effects are shown to cause CFRP de-bonding and loss of load carrying capabilities under static or low cyclic loading. Two series of RC beams with CFRP strengthening system are constructed and designed to fail in shear and flexural failures, respectively, under static loading. Repeated loading tests are conducted according to various loading ranges and loading cycles, and the experimentally determined fatigue properties are discussed. The test results show that it is possible to eliminate the debonding modes for longitudinally bonded CFRP using a U-wrap CFRP combination. The fatigue loads tested showed a significant effect on concrete rather than the CFRP system especially for the strengthened beams bearing a higher shear level. Moreover, the proposed equation to fit the testing S–N curve and the discussion of the stress in the component materials could be used for fatigue life predictions of beams with CFRP strengthening systems.     

Contribution to the understanding of the mechanical behaviour of CFRP-strengthened RC beams subjected to fire: Experimental and numerical assessment

Recent experimental tests and numerical simulations about the fire resistance behaviour of CFRP-strengthened RC beams proved that CFRP strengthening systems are able to attain considerable fire endurance, provided that adequate fire protection systems are used. In a fire event, even though a CFRP laminate may rapidly debond from the central part of the beam in which it is installed, if sufficiently thick insulation is applied in the anchorage zones, the laminate transforms into a “cable” fixed at the extremities, thus maintaining a considerable contribution to the mechanical response of the strengthened beam. This paper presents experimental and numerical investigations on CFRP-strengthened RC beams with the objective of understanding in further depth their fire resistance behaviour, namely the influence of the above mentioned “cable” mechanism on the mechanical response of the beams. The experimental campaign, performed at ambient temperature, comprised 4-point bending tests on RC beams strengthened with CFRP laminates according to either the EBR or the NSM techniques, in both cases fully or partially (only at the anchorages, thus simulating the cable mechanism) bonded to the soffit of the beams. For the test conditions used in this study, for both types of strengthening systems, partially bonding the CFRP laminates did not affect the stiffness of the beams and caused only a slight reduction of their strength (6–15%). The numerical study comprised the simulation of the structural response of all beams tested. Non-linear finite element models were developed in Atena commercial package, in which a smeared cracked model was adopted to simulate concrete and appropriate bond-slip constitutive relations were defined for the CFRP-concrete interfaces. A very good agreement was obtained between experimental data and numerical results, providing further validation to the “cable” mechanism and the possibility of taking it into account when designing fire protection systems for CFRP-strengthened RC beams.     

Load–deflection behavior of RC beams strengthened with GFRP sheets subjected to different environmental conditions

An investigation to examine the durability of reinforced concrete beams strengthened with glass fiber reinforced polymer (GFRP) laminates is performed. A total of 84 beam specimens were prepared for this study. The performance of these specimens was assessed through evaluating the flexural capacity and load–deflection relationships of the beams after placing them in different environments directly or indirectly with simulated field condition for a specified period of time. The specimens were divided into six categories which include controlled laboratory environment (unexposed category), outside environment (direct exposure to hot–dry field conditions), wet–dry normal water environment, wet–dry saline (NaCl) water environment, and wet–dry alkaline (NaOH) environment. Each category consisted of unstrengthened and strengthened beams. Furthermore, some of the specimens of the hot–dry field exposure were coated with protection paint against ultra violet (UV) rays. The specimens of different wet–dry environments were exposed to a time cycle of two weeks inside the solution and two weeks outside the solution. The test results carried out after 6, 12 and 24 months of exposure to different environmental conditions, show that none of the aforesaid environmental conditions have a noticeable influence on the flexural strength of the beams.     

Application of a self-compacting ultra-high-performance fibre-reinforced concrete to retrofit RC beams subjected to repeated loading

The aim of this paper is to describe the performance of reinforced concrete (RC) beams retrofitted with a self-compacting ultra-high-performance fibre-reinforced concrete (UHPFRC) under three-point bend cyclic loading. It is found that retrofitting the RC beams with a thin UHPFRC strip on the tension face increases their endurance limit under a non-zero mean stress cyclic loading from approximately 40% to approximately 60% of their static three-point flexural strength. Moreover, the retrofitted beams behave as a composite structure, with no delamination of the retrofit strip being observed in any of the fatigue tests.     

Behavior of RC T-section beams strengthened with CFRP strips,subjected to cyclic load

This paper presents results of an experimental investigation on T-section reinforced concrete (RC) beams strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) strips. Specimens, one of which was the control specimen and the remaining six were the shear deficient test specimens, were tested under cyclic load to investigate the effect of CFRP strips on behavior and strength. Five shear deficient specimens were strengthened with side bonded and U-jacketed CFRP strips, and remaining one tested with its virgin condition without strengthening. The type and arrangement of CFRP strips and the anchorage used to fasten the strips to the concrete are the variables of this experimental work. The main objective was to analyze the behavior and failure modes of T-section RC beams strengthened in shear with externally bonded CFRP strips. According to test results premature debonding was the dominant failure mode of externally strengthened RC beams so the effect of anchorage usage on behavior and strength was also investigated. To verify the reliability of shear design equations and guidelines, experimental results were compared with all common guidelines and published design equations. This comparison and validation of guidelines is one of the main objectives of this work. The test results confirmed that all CFRP arrangements differ from CFRP strip width and arrangement, improved the strength and behavior of the specimens in different level significantly.     

Seismic performance of post-mainshock FRP/steel repaired RC bridge columns subjected to aftershocks

This study compares the performances of three types of repair jackets on mainshock (MS) earthquake-damaged RC bridge columns subjected to aftershock (AS) attacks. These repair jackets include fiber reinforced polymers (FRP), thick steel, and thin steel wrapped with prestressing strands. Results obtained from incremental dynamic time history analyses on refined numerical finite element bridge models were utilized to evaluate the efficacy of different repair jackets application on the post-MS collapse safety of RC bridges subjected to AS attacks of various intensities. Numerical results indicated that the three repair jackets can effectively improve the bridge collapse capacity by approximately 20% under severe MS-severe AS even though they cannot restore the initial stiffness of damaged columns. Repair jackets for the severe MS-damaged columns were ineffective under moderate AS events and thus not required. Steel repair jackets exhibited higher energy dissipation under MS–AS sequences than FRP jackets. In the case of FRP jackets, bidirectional fiber wraps are recommended for plastic hinge confinement of MS-damaged bridge columns subjected to aftershocks.     

Interaction between CFRP-repair and initial damage of wide-flange steel beams subjected to three-point bending

This paper presents the interaction between carbon fiber reinforced polymer (CFRP) composites and the level of damage in steel beams. An experimental program is conducted with three different sizes of notches at midspan of the beams to simulate initial damage prior to repair. A three-dimensional finite element analysis (FEA) is conducted to predict experimental behavior. Unlike existing predictive models that assume perfect bond between CFRP and steel substrate, the proposed modeling approach explicitly accounts for the bond–slip behavior of CFRP–steel interface. CFRP-repair improves the load-carrying capacity of damaged beams that have failed by crack propagation across the steel section with wide opening of the notch. For repaired beams; stress concentrations at a damage location result in local debonding of the CFRP sheet, followed by complete debonding failure of the sheet. These failure modes are found to be independent of the level of initial damage (notch depth). CFRP-repair delays crack-formation of the repaired beams; however, such an effect is not significant once a crack propagates towards the upper flange. The level of initial damage influences debonding propagation rates of the CFRP. Two distinct bond–slip responses of the CFRP–steel interface are experimentally observed.     

HPFL加固受火RC梁抗剪性能研究

 对高性能复合砂浆钢筋混凝土加固受火RC梁的抗剪承载力进行理论推导,并结合实验数据和工程实例验证理论公式的合理性．结合国内外对火灾后钢筋混凝土结构中混凝土和钢筋强度变化的研究,考虑火灾和火灾后冷却条件对混凝土和钢筋强度及性能的影响,提出计算模型．假定钢筋和混凝土之间无相对滑移,忽略混凝土的抗拉强度,不考虑温度—应力的耦合作用,采用等效截面法得到等效截面为T形截面,以桁架 拱模型和软化桁架理论为基础,结合极限平衡原理,考虑拉应变存在条件下混凝土抗压强度的软化．分析结果表明,推导所得的计算公式与试验数据比较吻合,高性能复合砂浆钢筋网加固方法能使梁的抗剪承载力得到显著提高,能满足实际工程的应用．